Optoelectronic device

ABSTRACT

An optoelectronic device, including: a rib waveguide, the rib waveguide including: a ridge portion, which includes a temperature-sensitive optically active region, and a slab portion, positioned adjacent to the ridge portion; the device further comprising a heater, disposed on top of the slab portion wherein a part of the heater closest to ridge portion is at least 2 μm away from the ridge portion. The device may also have a heater provided with a bottom cladding layer, and may also include various thermal insulation enhancing cavities.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 16/281,035, filed Feb. 20, 2019, entitled “OPTOELECTRONICDEVICE”, which claims priority to United Kingdom Patent Application No.GB 1802763.1, filed Feb. 21, 2018, entitled “OPTOELECTRONIC DEVICE”; theentire contents of all of the documents identified in this paragraph areincorporated herein by reference.

FIELD

The present invention relates to heaters in optoelectronics, andparticularly to heaters in electro-absorption modulators.

BACKGROUND

Optoelectronic devices, and particularly electro-absorption modulators(EAMs) can be temperature sensitive. For example, the working wavelengthof an EAM may vary considerably when the temperature of the devicevaries. The physical mechanism underlying this is that the band-edgewavelength of the material forming the EAM can have a temperaturedependence.

This temperature dependence can be beneficial, for example whenoperating in a coarse wavelength division multiplex (CWDM) mode. Howeverin such operational modes the temperature of the device must beaccurately controlled.

In conventional devices, a heater is placed immediately adjacent to oron top of (relative to the substrate) the EAM. In such devices a severetemperature gradient can form across the EAM, which may significantlydegrade the performance of the EAM.

SUMMARY

Generally the disclosure relates to the provision of heaters in anoptoelectronic device in a manner providing more uniform heating, and tothe thermal isolation thereof so as to increase efficiently. In oneaspect, some embodiments of the invention relate to providing cavitiesor trenches so as to thermally isolate the heater and optically activeregion.

In a first aspect, some embodiments of the invention provide anoptoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

the device further comprising a heater, disposed on top of the slabportion wherein a part of the heater closest to ridge portion is atleast 2 μm away from the ridge portion.

By placing the heater at least 2 μm away from the ridge portion, a farmore uniform temperature can be established within the ridge portion ofthe waveguide which contains the temperature-sensitive optically activeregion. In some examples, the part of the heater closest to the ridgeportion is at least 3 μm away from the ridge portion.

In a second aspect, some embodiments of the invention provide anoptoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;        the device further comprising a heater, disposed in an epitaxial        crystalline cladding layer which is located beneath the slab        portion.

By providing the heater within an epitaxial crystalline cladding layer,better temperature uniformity can be obtained. Moreover the increase inthe footprint of the device can be minimized whilst providing a heater.The heater also does not use the exposed surface area of the device, andmay not suffer from electromigration or self-Joule heating (which areboth failure mechanisms in heaters).

The heaters of the first and second aspects can allow anelectro-absorption modulator included in the rib waveguide to operateacross a range of wavelengths. For example, when the optically activeregion provides an electro-absorption modulator, the device may beoperable from a wavelength of at least 1450 nm to no more than 1610 nm,and, in some embodiments, at least 1550 nm to no more than 1610 nm. Thismay allow the use of a coarse wavelength division multiplexing scheme.The optically active region may be formed of Si_(x)Ge_(1-x) where0.005≤x≤0.01 and, in some embodiments, where 0.005<x<0.01.

In a third aspect, some embodiments of the invention provide anoptoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;        the device further comprising:        a heater, for heating the temperature-sensitive optically active        region;        a bottom cladding layer, disposed adjacent to the slab portion;        and a thermal isolation trench, wherein the thermal isolation        trench is positioned adjacent to the bottom cladding layer.

The thermal isolation trench operates to thermally isolate the heaterand optically active region from the remainder of the device, and so canincrease the efficiency of the heater. As a result, less energy may berequired to maintain the optically active region at a desiredtemperature.

In a fourth aspect, some embodiments of the invention provide anoptoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

wherein the device further comprises:

a heater, for heating the temperature-sensitive optically active region;

a bottom cladding layer, disposed adjacent to the slab portion; and

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

The thermal isolation cavity operates to thermally isolate the heaterand optically active region from the remainder of the device, and so canincrease the efficiency of the heater. As a result, less energy may berequired to maintain the optically active region at a desiredtemperature.

In a fifth aspect, some embodiments of the invention provide anoptoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;        wherein the device further comprises:        a heater, for heating the temperature-sensitive optically active        region; and        an electrode, electrically connected to the either the ridge        portion or the slab portion, and a heater positioned between the        electrode and the ridge portion;        wherein the electrode includes at least one thermal isolation        cavity.

The or each thermal isolation cavity in the electrode operates tothermally isolate the heater and optically active region from theremainder of the device, and so can increase the efficiency of theheater. As a result, less energy may be required to maintain theoptically active region at a desired temperature.

By rib waveguide, it may be meant that the waveguide acts to confine anoptical mode of the waveguide within the ridge portion of the waveguide.Alternatively, the device may be a ridge waveguide by which it may bemeant that the optical mode of the waveguide is confined within the slabportion of the waveguide and the ridge portion may act to guide lightpassing through the waveguide. As a further alternative, it may be meantthat the optical mode is confined to both the ridge portion and the slabportion. In all aspects discussed above, the bottom cladding layer maybe an epitaxial crystalline cladding layer. By adjacent, it may be meantthat the slab portion of the waveguide is immediately adjacent to theridge portion. The entire rib waveguide may be considered as forming aninverted ‘T’ shape, where the leg of the T is provided by the ridgeportion and the bar of the T is provided by the slab portion. The slabportion may be adjacent to the ridge portion in a directionperpendicular to the guiding direction of the waveguide. The ridgeportion may be considered to be the portion of the waveguide whichextends from the slab portion in a direction away from a siliconsubstrate. The slab portion may also include the portion of thewaveguide directly beneath (relative to an upper surface of the device)the ridge portion. Alternatively, the slab portion can be considered tohave two sub-portions, either side of the ridge portion such that theridge portion bisects the overall slab portion.

Optional features of some embodiments of the invention will now be setout. These are applicable singly or in any combination with any aspectof embodiments of the invention.

A width of a first region of the heater may taper from a first width toa second width in a direction parallel or substantially parallel to aguiding direction of the rib waveguide. The width of a second region ofthe heater increases from the second width to the first width along thedirection parallel or substantially parallel to the guiding direction ofthe rib waveguide. The tapering region may be used to decrease thejunction current density below a threshold, and so help in avoidingelectromigration (a cause of failure in some heaters).

The heater may be formed from any one of: Ti, TiN, TiW, NiCr, or W, andmay, in some embodiments, be formed of either Ti or TiN.

The heater may comprise plural metal strips, connected at one end to anadjacent metal strip so as to form a serpentine form. By doing so, theelectrical length of the heater can be increased whilst not increasingthe footprint of the heater in the device. This increase in electricallength can increase the electrical resistivity of the heater, which canreduce the current density in the waveguide. If the serpentine heater ispowered by a constant current source, it may display an increase ingenerated heat as compared to a non-serpentine heater. The heater maycomprise at least 2 metal strips and no more than 9 metal strips, and,in some embodiments, at least 2 and no more than 5 metal strips. Theheater may include a first and second electrode for the heater, whichare electrically connected to the heater on the same side. By same side,it may be meant that the heater can be generally rectangular, and thatthe electrodes may electrically connect to the heater on a same side ofthe rectangle. Each metal strip may have a width of at least 0.5 μm andno more than 15 μm, and, in some embodiments, may have a width of atleast 2.0 μm and no more than 4.0 μm. A gap between adjacent metalstrips may have a width of at least 0.5 μm and no more than 10 μm, and,in some embodiments, may have a width of at least 1.0 μm and no morethan 2.0 μm.

The heater may be disposed above an electrical contact for the slabportion and separated therefrom by an insulator. By doing so, theoverall footprint of the device may be maintained whilst ensuring thatthe heater does not electrically interfere with the slab portion.

The device may include a second heater, identical or substantiallyidentical to the first and disposed on an opposing side of the ridgeportion. By opposing side, it may be meant that the slab portion has tworegions one on a first side of the ridge portion and one on a secondside of the ridge portion. The opposing side may be the second side, andthe first heater may be disposed on the first side. By identical, it maymean that the second heater is structurally identical to the first butmirrored in a plane aligned with the ridge portion.

The heater may comprise a doped region of the epitaxial crystallinecladding layer, or a doped region of a silicon-on-insulator layerdisposed beneath the slab portion of the waveguide. The epitaxialcrystalline cladding layer may be formed of silicon. The doped region ofthe epitaxial crystalline cladding layer may extend in a directionparallel or substantially parallel to the guiding direction of the ribwaveguide. The doped region may have a width of at least 1 μm and nomore than 30 μm, and, in some embodiments, has a width of at least 3 μmand no more than 20 μm. The doped region may have a height of at least0.1 μm and no more than 0.3 μm, and, in some embodiments, has a heightof at least 0.15 μm and no more than 0.2 μm. The doped region may have adoping concentration of at least 1×10²⁰ cm⁻³ and no more than 2.5×10²⁰cm⁻³. The device may further include an undoped region of the epitaxialcrystalline cladding layer, the undoped region being located between thedoped region and the slab portion.

The thermal isolation trench may be filled with either air or silicondioxide, and is, in some embodiments, filled with air. The thermalisolation trench may have a width of at least 0.5 μm and no more than2.0 μm. The device may include plural thermal isolation trenches, whichare arranged around a periphery of the slab portion. By periphery, itmay be meant that the thermal isolation trenches are disposed around anoutermost edge of the slab portion. The outermost edge may be the onefurthest from the ridge portion, as measured in the plane of the device.For example, the slab portion may be generally rectangular, and so thethermal isolation trenches would be disposed along the edges of therectangle.

The device may further include a buried oxide layer, disposed adjacentto a lower surface of the bottom cladding layer, wherein the thermalisolation cavity is located on an opposing side of the buried oxidelayer and is adjacent to a silicon substrate. The thermal isolationcavity may have a width which is larger than a width of the slabportion. Between the bottom cladding layer and the slab portion theremay be a crystalline rare earth oxide (CREO) or rare earth oxide (REO)layer. This CREO or REO layer, and the bottom cladding layer, may have acrystalline orientation of (111).

The electrode for the modulator may comprise plural thermal isolationcavities in an array, wherein the array extends in a direction parallelor substantially parallel to the guiding direction of the rib waveguide.The array may extend for a length of at least 50 μm and no more than 100μm and, in some embodiments, for a length of at least 50 μm and no morethan 70 μm. The electrode may comprise at least 2 cavities and no morethan 30 cavities, and, in some embodiments, comprises at least 5 and nomore than 10 cavities. The or each cavity in the electrode may have alength of at least 2 μm and no more than 30 μm, and, in someembodiments, has a length of at least 5 μm and no more than 10 μm. Theor each cavity in the electrode may have a width of at least 1 μm and nomore than 10 μm, and, in some embodiments, at least 3 μm and no morethan 5 μm. A gap between adjacent cavities in the electrode may have alength of at least 1 μm and no more than 20 μm and, in some embodiments,a length of at least 1 μm and no more than 3 μm. The or each cavity inthe electrode may be filled with air or SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows a top-down view of an optoelectronic device;

FIG. 2A(i) shows a cross-sectional view of the device in FIG. 1 alongthe line A-A′;

FIG. 2B(i) shows a top-down view of a heater for an optoelectronicdevice;

FIG. 2C shows a cross-sectional view of the device in FIG. 1 along theline B-B′;

FIG. 2A(ii) shows a variant cross-sectional view of the device in FIG. 1along the line A-A′;

FIG. 2B(ii) shows a variant top-down view of a heater for anoptoelectronic device;

FIG. 3 shows a top-down view of a variant optoelectronic device;

FIG. 4A shows a top-down view of a variant optoelectronic device;

FIG. 4B shows a top-down view of a variant optoelectronic device;

FIG. 5A shows a cross-sectional view of the device in FIG. 4A along theline A-A;

FIG. 5B shows a cross-sectional view of the device in FIG. 4A along theline B-B′;

FIG. 5C shows a top-down view of a variant heater for an optoelectronicdevice;

FIG. 6A shows a top-down view of a variant optoelectronic device;

FIG. 6B shows a cross-section of the device in FIG. 6A along the lineA-A′;

FIG. 6C shows a top-down view of the variant heater in 6A for theoptoelectronic device;

FIG. 7A-7C show, respectively, top-down views of variant heater elementsfor an optoelectronic device;

FIGS. 8A and 8B show, respectively, cross-sectional views of variantoptoelectronic devices;

FIG. 9 shows two examples of thermal isolation trenches;

FIG. 10 shows a top-down view of a variant optoelectronic device;

FIG. 11A shows a cross-sectional view of a variant optoelectronicdevice;

FIG. 11B shows a top-down view of the optoelectronic device of FIG. 11A;

FIG. 12A shows a cross-sectional view of a variant optoelectronicdevice, and 12B shows an enlarged section of the device shown in FIG.12A;

FIG. 13 shows a top-down view of a variant optoelectronic device;

FIG. 14 shows an enlargement of the top-down view of FIG. 13;

FIG. 15A shows a top-down view of a variant optoelectronic device;

FIG. 15B shows a cross-sectional view of the device of FIG. 15A alongthe line A-A′;

FIG. 16 shows a cross-sectional view of a variant of the device of FIG.15A along the line A-A′;

FIG. 17A shows a top-down view of a variant optoelectronic device;

FIG. 17B shows a cross-sectional view of the device of FIG. 16A alongthe line A-A′; and

FIGS. 18A-18D are plots showing the results of a simulation of variousdevices; and

FIGS. 19A-19D are plots showing the results of a simulation of variousdevices.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

FIG. 1 shows a top-down review of an optoelectronic device 100. An inputwaveguide 101 guides light into a rib waveguide which comprises a ridgeportion 102 and a slab portion 103 atop of a silicon-on-insulator layer202. The ridge portion may, for example, include or provide anelectro-absorption modulator (EAM) or photodiode structure. Depending onthe nature of the ridge portion of the rib waveguide, light may exit thedevice through output waveguide 104. Connected to the rib and slabportions are electrical pads 105 which may be formed of titanium oraluminium. Generally the electrodes are formed of 1 μm thick aluminium,and may contain a 10 nm thick titanium layer between the aluminium andthe slab as an electrical barrier. Disposed on top of a part of the slabportion 103, is a heater 106. The heater is formed of a generallyelongate metal strip, located no closer than 2 μm to the ridge portion102 of the rib waveguide. The heater is connected to junctions 107 ateither end, which respectively connect ends of the heater 106 to metaltraces 109. The metal traces 109 then connect to the electrical pads 108for the heater. In this example, the electrical pads are formed ofaluminium and the heater is formed of Ti or TiN. The metal traces mayalso be formed of aluminium, and would generally introduce a resistanceof less than 1Ω, for example around 0.6Ω. A total length of each metaltrace may be around 400 μm, a width of each metal traces may be around20 μm and the metal traces may have a height of around 1 μm. Thedimensions of some parts of the optoelectronic device are indicated inthe figure.

A change of 35° C. within the rib waveguide may cause a wavelength shiftof around 27 nm or 30 nm. This change in average temperature within therib waveguide may be referred to as ΔT. A heater which is an embodimentof the present invention had the following properties:

w t Efficiency R I V_(in) Power T_(heater) 2 μm 100 nm 0.37° C./mW 80 Ω34 mA 2.7 V 94 mW 385° C.

where w is the width of the heater. R is the electrical resistance; I isthe current, J is the current density, V_(in) is the input voltage, andT_(heater) is the temperature, all as measured within the heater whenΔT=35° C. The heater thickness, t, may be at least 30 nm and no morethan 500 nm and, in some embodiments, at least 50 nm and no more than200 nm. The values in this table are for an example where there is noburied oxide layer beneath the slab.

It is worth discussing at this stage, the principal directionsassociated with the device 100. As indicated by y, one principaldirection is generally aligned with a guiding direction of the input andoutput waveguides 101 and 104. At 90 degrees to this, but still withinthe plane of the device is principal direction x. And at 90 degrees toboth directions y and x is direction z extending out of the plane of thedevice.

As shown in FIG. 2A(i), which is a cross-section of the device shown inFIG. 1 along the line A-A′, heater 106 extends across a width of theelectrical pad 105 for the waveguide and a part of the slab portion 103to connect at respective ends to the metal traces 109. The electricalpad 105 may be electrically connected to the slab portion 103 which maybe doped, for example n++ doped. The electrical pad in this example isaround 1 μm tall, i.e. as measured in the z direction. As can be seen inthis cross-section, the device further comprises a buried oxide (BOX)layer 203 (which may be 400 nm thick) which is above a silicon substrate204, as well as a silicon-on-insulator layer 202 (which may be 400 nmthick) which is above the buried oxide layer. Generally these devicesare covered by upper cladding layer 201, which may be formed of silicondioxide (SiO₂) and may be 500 nm thick. The upper cladding layer may bearound 0.5 μm thick. This upper cladding layer can be used forpassivation i.e. to prevent oxidation. As can be seen, a central portionof the heater extends along direction y for approximately 38 μm with therespective connectors adding around 10 μm of length to the heater. Belowthe silicon substrate 204 may be a further buried oxide layer, and belowthe further buried oxide layer there may be a further silicon substratei.e. the entire device is provided on a DSOI (doublesilicon-on-insulator) wafer.

FIG. 2B(i) is a partial top-down view of the heater 106. As can be seenin region 106 a of the heater, a width of the heater as measured indirection x tapers from around 17 μm to 2 μm in a direction parallel tothe guiding direction (i.e. direction y) the width of the heater thenincreases from 2 μm to around 17 μm in region 106 b. These taperedregions 106 a and 106 b allow the heater to connect to the metal traces109 whilst ensuring that the current density is not too high in anygiven region. FIG. 2C shows a cross-sectional view of the device in FIG.1 along the line B-B′. As can be seen, the electrode 105 for thewaveguide are disposed underneath the heater 106, and electricallyconnect to doped regions of the slab 103. In some examples, the buriedoxide layer 203 below the slab 103 may be etched away and replaced withan epitaxial crystalline semiconductor e.g. Si or SiGe.

FIGS. 2A(ii) and 2B(ii) show a variation of the device shown in FIGS.2A(i) and 2B(i), and so like features are indicated by like referencenumerals. A difference between the two devices is that the aluminiumtrace 109A for the heater includes a portion which extends away from thesilicon-on-insulator layer 202 and then over an upper surface of theheater 106. The junction 107A and electrical connection from the trace109 to the heater 106 is therefore made on an upper surface of theheater. Whereas in FIGS. 2A(i) and 2B(i), the electrical connection isbetween a lower surface of the heater and the trace. The electricaljunction 107A described in relation to FIGS. 2A(ii) and 2B(ii) may bemore reliable than the equivalent disclosed in FIGS. 2A(i) and 2B(i).

A variant optoelectronic device shown in FIG. 3, where two identicalheaters 106A and 106B are disposed on either side of the rib waveguide.The heaters are identical except that they are mirrored in a planealigned with the ridge portion of the waveguide. The table below givesan indication of the difference two heaters (as shown in FIG. 3) makesas compared to a single heater

TABLE 1 Under- Efficiency R I V_(in) Power T_(heater) cut Trench N °heaters (° C./mW) (Ω) (mA) (V) (mW) (° C.) N N 1 0.37 80 34 2.7 94 385 NN 2 0.37 80 24 1.9 94 220 Y Y 1 1.02 80 21 1.6 34 180 Y Y 2 1.02 80 151.2 34 119

Where the heater had a thickness, t, of 100 nm (measured in the zdirection) and a narrowest width, w, of 2 μm (measured in the xdirection). In the table, R is the electrical resistance of the heater,I is the input current, V is the input voltage, and T_(heater) is thetemperature of the heater. The values for I, V_(in), Power, andT_(heater) are for when ΔT, the change in average temperature of thewaveguide, is 35° C. The values in Table 1 are for examples where aburied oxide layer located beneath the silicon substrate 204 has athickness of around 0.4 μm, and where the buried oxide layer 203 locatedbetween the silicon substrate and the slab 103 has been replaced withsilicon. Of note, is that by including a second heater the temperatureincrease within each heater is around half that of the single heaterembodiment. Also, the input voltage required to both heaters in the twoheater embodiment is around 70% of the voltage required in the singleheater embodiment. The parameters ‘under-cut’ and ‘trench’ indicate thepresence of a cavity or thermal isolation trench as discussed below.

FIG. 4A shows a variant optoelectronic device, where like features areindicated by like reference numerals. A difference between thisoptoelectronic device and that shown in FIG. 1 is that the heater 406Acomprises plural metal strips which extend in direction y and areconnected at one end to an adjacent metal strip. In this example, theheater 406A is formed of four metal strips and forms a generallyserpentine shape. By doing so, the electrical length of the heater canbe increased and therefore the available heat output will similarlyincrease. This increase in electrical length can increase the electricalresistivity of the heater, which can reduce the current density in thewaveguide. If the serpentine heater is powered by a constant currentsource, it may display an increase in generated heat as compared to anon-serpentine heater. As with heater 106 in FIG. 1, no part of heater406A is closer than 2 μm to the ridge portion of the waveguide 102. Alsoshown in this figure is that the electrical traces 109 both contact theheater 406A on a same side of the heater e.g. the side closest to theinput waveguide 101. This can serve to further reduce the footprint forthe device including the heater. FIG. 4B shows a variation of the deviceshown in FIG. 4A. In this figure, the heater 406B comprises an oddnumber of metal strips, and therefore metal traces 109 connect to theheater at opposing ends relative to the input waveguide 102.

Generally, the process for flow providing such devices comprises thesteps of: (i) depositing the electrical pad and metal traces for theheater at the same time as depositing the electrical pad for thewaveguide; (ii) depositing the upper cladding layer and etching vias forconnection to the electrical pad and metal traces for the heater; (iii)depositing an at least 50 nm and no more than 200 nm thick heater layer,for example a 100 nm thick heater layer, and patterning said layer; and(iv) depositing an oxide of around 0.5 μm in thickness for passivatingthe heater layer. The heater may be provided by depositing titanium.

FIG. 5A is a cross sectional view of the device shown in FIG. 4A alongthe line A-A′. It is similar to the cross section shown in FIG. 2A(i)and so like features are indicated by like reference numerals. Notablechanges include that the heater now comprises plural metal strips whichhave a total width as measured in the x direction of around 17 μm. Thiswidth is one example, however many other total widths can be used.Generally, the width can be calculated as w_(total)=n×w_(strip)+(n−1)×g,where n is the number of strips, w strip is the width of a single strip,and g is the width of the gap between adjacent strips. Also of note, butwhich was also present in FIG. 2A(i), is an insulating layer 407 whichserves to electrically isolate the heater 406 from the electrical pad105 of the waveguide. A via exists in the upper cladding layer 201 sothat the electrode 105 for the waveguide can electrically contact theslab region. This via has a width of around 18 μm and a length of around38 μm.

FIG. 5B is a cross-sectional view of the device shown in FIG. 4A alongthe line B-B′. Here the extension of the heater 406 can be more easilyseen. The connection from metal trace 109 to the heater 406 is over adistance of no more than 5 μm. In this example, the material comprisingthe heater extends away from the core region of the heater and contactsthe metal trace 109 through a separate via in the upper cladding layer201. FIG. 5C is a partial top-down view of the heater 406 as included inthe device shown in FIG. 4A. As can be seen, the core region of theheater is defined by a rectangular region of width 17 μm and length 38μm. The core region of the heater may have a length which is between 2μm and 4 μm smaller than that of the via for the EAM electrode-SiGeslab. The core region of the heater may have a width which is between 1μm and 5 μm smaller than half the width of the via. Whilst the heaterhere has a width of 17 μm, as mentioned above other values may be used.Generally the width of the heater is defined as:w_(heater)=n×w_(strip)+(n−1)×g.

FIG. 6A shows a partial top-down view of a further variant device. Here,the heater 506 is similar to the heater 406 disclosed previously exceptthat the metal traces 109 extend away from the silicon-on-insulatorlayer 202 in the z direction, and then further onto the slab region.This is shown most clearly in FIG. 6B, where metal traces 109 contactthe heater 506 through a via in the upper cladding layer 201 which isabove the electrode 105 for the waveguide. FIG. 6C shows a partialtop-down view of the heater 506. Here, the core region of the heaterstill is defined by a rectangular region of width 17 μm and length 38μm. However, in this example the metal traces 109 extend part of the wayinto this core region.

Generally, the process flow for providing these devices includes (i)depositing the electrical contact for the waveguide, patterning it, anddepositing an oxide cladding; (ii) depositing a 1 μm thick electricalcontact and metal traces for the heater, which may be formed fromaluminium, and patterning; (iii) depositing a heater layer andpatterning, the heater layer may be at least 50 nm and no more than 200nm thick, for example 100 nm thick; and (iv) depositing an oxide forpassivating the heater layer. Steps (ii) and (iii) may be interchanged.

FIGS. 7A-7C show partial top-down views of three variant heaters. Ineach, the number of metal strips 810 varies. For example, in the exampleshown in FIG. 7A there are eight metal strips. As the overall footprintof the heater remains the same, e.g. 38 μm by 17 μm, the thickness ofthe gap g vary as n the number of strips varies. Generally the heatercan be more efficient if n is minimised. Varying these propertieschanges the parameters as set out in table 2 below:

TABLE 2 Under- g Efficiency R I V_(in) Power T_(heater) cut trench n(μm) (° C./mW) (Ω) (mA) (V) (mW) (° C.) N N 1 N/A 0.37 80 34 2.7 94 385N N 3 1.5 0.33 246 21 5.1 105 200 N N 5 1.5 0.30 412 17 6.8 114 155 Y Y1 1.5 1.02 80 21 1.6 34 180 Y Y 3 1.5 0.98 246 12 3.0 36 109 Y Y 5 1.50.94 412 10 3.9 37 93

where ‘Under-cut’ and ‘trench’ indicate the provision of an under-cutand trench as discussed below; I is the current passing through theheater when ΔT, the average increase in the temperature of thewaveguide, is 35° C.; J is the current density when ΔT is 35° C.; andT_(max) is the maximum temperature of the heater when ΔT is 35° C. Theexamples above included a heater only on one side of the waveguide.There was no buried oxide layer located immediately between the slabportion of the waveguide and the silicon layer, as discussed previouslyit was replaced with an epitaxial crystalline cladding layer. A buriedoxide layer below the silicon layer 605 had a thickness of 0.4 μm, andthe heater had a thickness of 100 nm (measured in the z direction) and awidth of 2 μm. As discussed above, the width of the heater is generallya function of n, and so the width of the heater is not necessarilyconstant for all examples in Table 2.

Further characterization was performed by varying the closest distance Dbetween the heater and the ridge portion of the waveguide, as shown inthe table below:

TABLE 3 Under- D Efficiency I V_(in) Power T_(heater) cut trench (μm) (°C./mW) (mA) (V) (mW) (° C.) N N 3 0.37 34 2.7 94 385 N N 5 0.34 36 2.9103 427 N N 10 0.30 38 3.1 117 482 Y Y 3 1.02 21 1.6 34 180 Y Y 5 0.9921 1.7 35 188 Y Y 10 0.94 22 1.7 37 197The examples above included a heater only on one side of the waveguide.There was no buried oxide layer located immediately between the slabportion of the waveguide and the silicon layer, as discussed previouslyit was replaced with an epitaxial crystalline cladding layer. A buriedoxide layer below the silicon layer 605 had a thickness of 0.4 μm, andthe heater had a thickness of 100 nm (as measured in the z direction)and a width of 2 μm. This resulted in a heater with an electricalresistance of 80Ω. I, V_(in), Power and T_(heater) were all measuredwhen ΔT=35° C.

FIG. 8A shows a cross sectional view of the device shown in FIG. 4Aalong the cross section A-A′. Shown in this view, are more details ofthe ridge portion 102, which includes a first doped region 601 andsecond doped region 602. The first doped region 601 of the ridge may ben-type doped, and the second doped region 602 of the ridge may be p-typedoped. The slab portion also comprises first doped region 603 and seconddoped region 604. The first doped region 603 of the slab may be n-typedoped, and the second doped portion 604 of the slab may be p-type doped.Of course the first and second doped regions of the slab and ridgeportions may have alternative dopant types. The first doped region 603and the second doped region 604 of the slab may be more heavily doped ascompared to the doped regions of the ridge.

Immediately below the slab portion is an epitaxial crystalline claddinglayer 605. The epitaxial crystalline cladding layer may be substantiallythe same as that disclosed in either U.S. 62/528,900 or U.S. Ser. No.15/700,055 the entire contents of which is incorporated herein byreference. In some embodiments, the original buried oxide layer in thesilicone-on-insulator wafer has been etched away, and replaced with anepitaxially grown crystalline structure (commonly a semiconductor). Asis shown clearly in this view of the device, the heater 406 is no closerthan 2 μm to the rib waveguide portion 102. Also shown are theelectrical pads 105 for connecting to the doped region 603 and 604 ofthe slab portion. Table 2 below shows the changes in the parameters setout in table 1 when there is an epitaxial crystalline cladding layer 605immediately below the slab portion:

TABLE 4 w g Efficiency ΔT/V_(in) R I ΔT_(vert) T_(metal) n (μm) (μm) (°C./mW) (° C./V) (Ω) (mA) (° C.) (° C.) 4 3.5 1.0 0.23 1.72 186 31 0.1474 6 2.0 1.0 0.23 1.06 490 19 0.14 76 8 1.2 1.0 0.24 0.72 1091 13 0.1580Where I, J, ΔT_(vert) (the vertical temperature differential in thewaveguide), and T_(metal) are when ΔT=10° C.

As can be seen from the table 4, the heaters in examples where there isno buried oxide are slightly less efficient than those with a buriedoxide layer. Asides from this, the heaters operated similarly thoseimplemented above a buried oxide layer.

A variant example is shown in FIG. 8B, which is the same cross-sectionalview as shown in FIG. 8A. In contrast to the embodiment shown in FIG.8A, this example includes thermal isolation trenches 701 located eitherside of the bottom cladding layer 605. The bottom cladding layer, asdiscussed above may be an epitaxial crystalline cladding layer. Thethermal isolation trenches 701 may be formed of either entirely silicondioxide, or a silicon dioxide outer wall defining a volume filled withair. Immediately below the bottom cladding area 605, is a further buriedoxide layer 704 on the other side of which is a cavity 702. This furtherburied oxide layer may also be present in the example shown in FIG. 8A,but in this example there would be a silicon substrate on the other sidethereof. The cavity 702 is characterised by being absent of any silicon,and is formed in between silicon side walls 703. The cavity 702, whichmay be referred to as an undercut, may be as wide as or wider than thewidth of the slab portion.

Both the thermal isolation trenches 701 and cavity 702 may act tothermally isolate the heater and rib waveguide portion from theremainder of the device. This insulation can enhance the efficiency ofthe heater, and also ensure a more uniform temperature distributionthrough the rib and slab portions.

As shown in the left-hand side of FIG. 9, the thermal isolation trenchmay either comprise a silicon dioxide outer wall 801 enclosing a volume802 filled with air; or, the thermal isolation trench may be entirelyfilled with silicon dioxide 801 as shown in the FIG. on the right. Thewidth of the trench may be from at least 0.5 μm to at most 2.0 μm. Ifthe width is ≤1.0 μm then the trench may be completely filled with SiO₂.If the width is >1.0 μm, the trench may have the volume filled with air.

By varying the design parameters of these devices, embodiments accordingto the present invention possess the following properties:

TABLE 5 BOX Efficiency I V_(in) Power T_(heater) (μm) Under-cut Trench(° C./mW) (mA) (V) (mW) (° C.) 0.4 N N 0.37 34 2.7 94 385 1.0 N N 0.4531 2.5 78 329 2.0 N N 0.55 28 2.3 64 282 3.0 N N 0.63 27 2.1 56 255 0.4N Y 0.41 33 2.6 86 359 1.0 N Y 0.53 29 2.3 66 289 2.0 N Y 0.67 26 2.0 52241 3.0 N Y 0.78 24 1.9 45 216 0.4 Y N 0.57 28 2.2 61 273 0.4 Y Y 1.0221 1.6 34 180In the examples disclosed in this table, there was no buried oxide layerpresent immediately below the slab portion of the waveguide. That buriedoxide layer was replaced with an epitaxial crystalline cladding layer asdiscussed above. Further, the heater comprised a single metal strip witha thickness of 100 nm, width of 2 μm and a length of 38 μm, which had anelectrical resistance of 80Ω. ‘BOX’ indicates the height (as measured inthe z direction) of the buried oxide layer 704 between the cavity 702and the slab portion of the waveguide. The column ‘Under-cut’ and‘Trench’ indicate if a trench or under-cut (also referred to as acavity) are present in the example. I, V_(in), Power, and T_(heater) areall given for a value of ΔT of 35° C.

Alternatively, there may be a buried oxide layer 203 which extendsbetween the slab portion of the waveguide 603 and thesilicon-on-insulator layer 605. Generally this buried oxide layer wouldbe around 0.4 μm thick (as measured in the z direction). Again, byvarying the design parameters of the device, devices according to someembodiments of the present invention possess the following properties:

TABLE 6 BOX Efficiency I V_(in) Power T_(heater) (μm) Under-cut Trench(° C./mW) (mA) (V) (mW) (° C.) 0.4 N N 0.59 27 2.2 60 276 1.0 N N 0.6626 2.1 53 252 2.0 N N 0.76 24 1.9 46 228 3.0 N N 0.84 23 1.8 42 213 0.4N Y 0.64 26 2.1 55 259 1.0 N Y 0.77 24 1.9 45 226 2.0 N Y 0.94 22 1.7 37197 3.0 N Y 1.06 20 1.6 33 180 0.4 Y N 0.77 24 1.9 45 225 0.4 Y Y 1.6316 1.3 21 139In the examples disclosed in this table, the buried oxide layer 203extends across the entire width of the device, and so is presentimmediately below the slab portion of the waveguide and has a width of0.4 μm. The heater comprised a single metal strip, with a thickness of100 nm and a width of 2 μm and had an electrical resistance of 80Ω. I,V_(in), Power, and T_(heater) are all given for a value of ΔT of 35° C.In the examples in both tables above, the heater length (L) was 38 μm.As is understood, the efficiency of the heater scales as 1/L and therequired power for a given ΔT scales as L.

FIG. 10 shows a further variant device, and like features are indicatedby like reference numerals. In contrast to previous devices, the deviceshown in FIG. 10 further includes a thermal guard ring 901 whichcomprises a plurality of thermal isolation trenches 701 as describedpreviously. These thermal isolation trenches extend around a peripheryof the slab portion 103 to thereby define the thermal guard ring.

FIG. 11A shows a further variant device, and like features are indicatedby like reference numerals. In contrast to previous devices, the heaterin this example comprises a doped region 1006 in the bottom claddinglayer 605. As discussed previously the bottom cladding layer 605 may bean epitaxial crystalline bottom cladding layer. In this example, thedoped heating region 1006 is less than 2 μm from the rib waveguide 102,although of course the skilled person will understand that (as inprevious examples) all parts of the heater may be at least 2 μm awayfrom the rib waveguide portion. In this example, the metal trace 109connecting the heater to the electrical pads extends through the uppercladding area so as to electrically contact the doped region 1006. Thedoped region comprising the heater may be a heavily doped region e.g.n++ or p++.

FIG. 11B is a top down view of the device shown in FIG. 11A. Here thedotted region indicated by reference 1006 shows the approximate locationof the doped heating element relative to the slab portion 103. Metaltraces 109 contact the doped region 1006 at either end of the buriedheater. This example also includes the thermal guard ring 901 asdiscussed above.

FIG. 12A shows a further example of the doped heater 1006 discussedabove. In this example both thermal isolation trenches 701 and cavity702 are present as discussed above. As can be seen more clearly in FIG.12A, there exists a portion of un-doped bottom cladding area 605 inbetween the first doped region 603 of the slab to electrically isolateit from the doped heater 1006. This un-doped portion of the bottomcladding layer may be around 400 nm thick. The efficiency of the heaterin this example is around 0.9° C./mW. Without the thermal isolationtrenches but with the cavity, the efficiency is around 0.35° C./mW.Without the cavity, but with the thermal isolation trenches, theefficiency is around 0.24° C./mW. Without either the cavity or thethermal isolation trenches, the efficiency of the heater is around 0.20°C./mW.

FIG. 12B shows an enlarged section of the device shown in FIG. 12A. Herethe width of the heater 1006 is indicated, as is its height T, and thethickness d of the undoped region. Varying some of the parameters of thedevice alters the properties as set out in table 4 below:

TABLE 7 T W Efficiency R I V_(in) Power T_(heater) (nm) (μm) (° C./mW)(Ω) (mA) (V) (mW) (° C.) 200 2 1.25 285 10 2.8 27 64 200 5 1.24 114 161.8 28 63 200 10 1.21 57 23 1.3 29 64 200 20 1.16 29 33 0.9 30 64 150 51.24 152 14 2.1 28 63 100 5 1.24 228 11 2.5 28 63

In these examples, the buried oxide layer below the heater 1006 had aheight as measured in the z direction of 0.4 μm, and all examplesincluded a cavity or under-cut as well as thermal isolation trenches.The parameters I, V_(in), Power, and T_(heater) are for where ΔT=35° C.As can be seen, the efficiency does not have a strong dependence on W orT. It was also seen that a near uniform temperature distribution wasachieved inside the waveguide (with a variation of less than 0.3° C. foran average ΔT of 35° C.). The possible values of W range from at least 1μm to no more than 20 μm and the possible values for T range from atleast 100 nm to no more than 300 nm. In some embodiments, W falls withinthe range of at least 2 μm and no more than 7 μm, and T falls within therange of at least 150 nm and no more than 200 nm. It was observed thatlarger values of W, T or a larger doping concentration resulted in asmaller resistance and so larger current. The resistivity for the dopedSi was measured as 3 Ω·μm for a doping concentration of around 10²⁰ cm⁻³at 300 K.

FIG. 13 shows a partial top-down view of a further variant device. Here,the electrodes for the waveguide 105 include one or more cavities 1101disposed therein. The cavities may be filled with either silicon dioxideor air (as is the case with the thermal isolation trenches discussedabove).

FIG. 14 shows an enhanced partial view of the electrode shown in FIG.13. Here the relative dimensions of the cavities are discussed. aindicates a length of each cavity, b indicates the gap between adjacentcavities, and c indicates the width of each cavity. The overall lengthof the electrode L imposes the following restraint: L=(N+1)×b+(N×a),where N is the number of cavities. L may take a value of at least 50 μmto no more than 100 μm, and, in some embodiments, may be at least 50 μmand no more than 70 μm. Correspondingly, n may be at least 2 and no morethan 30 or, in some embodiments, at least 5 and no more than 10. a maytake a value of at least 2 μm and no more than 30 μm, or, in someembodiments, at least 5 μm and no more than 10 μm. b may take a value ofat least 1 μm and no more than 20 μm, or, in some embodiments, at least1 μm and no more than 3 μm. c may take a value of at least 1 μm and nomore than 10 μm, or, in some embodiments, at least 3 μm and no more than5 μm. By providing such cavities, an increase in heater efficiency ofaround 30% may be achieved. This is chiefly due to the enhancement inthermal isolation with respect to the heater and waveguide.

FIGS. 15A and 15B show a further variant of the optoelectronic device.In FIG. 15A, a serpentine heater 406 formed of plural metal strips, islocated directly above the first doped region 603 of the slab portion.The closest part of this heater is at least 2 μm away from the ribwaveguide portion 102. The entire device sits within a cavity of asilicon-on-insulator layer which has a height (as measured from theburied oxide layer) of around 3 μm. A cross-section along the line A-A′is shown in FIG. 15B. As can be seen here, in between the epitaxialcrystalline cladding layer 605 and the slab portion is a crystallinerare earths oxide (CREO) or a rare earths oxide (REO) layer 1203 whichcan be used to further increase the crystalline quality of the slab whengrown. The layer 1203 may have a thickness of from at least 20 nm to nomore than 400 nm. The crystal lattice orientation of the epitaxialcrystalline cladding layer and the CREO or REO layer may be (111).Moreover, the device include heavily doped regions 1201 and 1202 whichconnect respectively to electrical pads 105. The heavily doped regionscan decrease the series resistance of the doped portions of both theslab and rib.

FIG. 16 shows a variant structure to that shown in FIG. 15B, thedifference here being that an upper cladding layer 201 completelyencloses the heater 406 on an upper surface.

FIGS. 17A and 17B show a further variant device. In contrast to theexample disclosed in FIG. 16, this device further includes the thermalguard ring 2101 (as discussed above), and also the cavity 702 located onan opposing side of the buried oxide layer to the bottom cladding layer605.

As regards to the heaters discussed above which are formed of pluralmetal strips, in some examples the heater may be formed of Ti or TiN inaccordance with the following:

TABLE 8 Dry Deposition time Dry etch time Deposition etch for 100 nm for100 nm Material rate rate thickness thickness Ti 5-6.5 nm/min @ 60 ~20minutes 1.7 minutes 500 W nm/min TiN   2.4 nm/min @ 60 ~42 minutes 1.7minutes 800 W nm/min

FIGS. 18A-18D are plots showing the results of a simulation of variousdevices according to some embodiments of the present invention, FIG. 18Ais a plot of efficiency against D, the minimum distance between theheater and the ridge portion, and FIG. 18B is a plot of power against Din order to obtain a ΔT value of 35° C. Whilst the efficiency decreaseswith increasing D, and the required power increases with increasing D, amuch increased temperature uniformity (ΔT_(vert)) can be observed inFIG. 18C. FIG. 18D is a plot of heater temperature against D in order toobtain a ΔT value of 35° C. The device simulated in FIGS. 18A-18D hadthe following parameters: one heater; =1; t=100 nm; no undercut, and notrench provided. The multiple lines on each plot indicate devices withvarying values of w.

FIGS. 19A-19D are plots showing results of a simulation of variousdevices according to some embodiments of the present invention. Theseplots mirror those shown in FIGS. 18A-18D. However, the devicessimulated for FIGS. 19A-19D were provided with an undercut and trench asdiscussed above. As can be seen then, when comparing these plots tothose in FIGS. 18A-18D, the efficiency is increased and the requiredpower decreases.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

LIST OF FEATURES

-   Optoelectronic device 100-   Input waveguide 101-   Ridge portion of rib waveguide 102-   Slab portion of rib waveguide 103-   Output waveguide 104-   Electrical pad for waveguide 105-   Heater 106, 406, 506, 1006-   Junction 107-   Electrical pad for heater 108-   Metal trace 109-   Upper cladding layer 201-   Silicon-on-insulator layer 202-   Buried oxide 203-   Silicon substrate 204-   Insulator layer 407-   1^(st) doped region of rib 601-   2^(nd) doped region of rib 602-   1^(st) doped region of slab 603-   2^(nd) doped region of slab 604-   Silicon layer 605-   Thermally insulating trench 701-   Thermally insulating cavity 702-   Silicon substrate 703-   SiO₂ wall 801-   Air cavity 802-   Thermal guard ring 901-   Cavity in electrical pad 1101

Clauses

1. An optoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

the device further comprising a heater, disposed on top of the slabportion wherein a part of the heater closest to ridge portion is atleast 2 μm away from the ridge portion.

2. The optoelectronic device of clause 1, wherein a width of a firstregion of the heater tapers from a first width to a second width in adirection substantially parallel to a guiding direction of the ribwaveguide.3. The optoelectronic device of clause 2, wherein the width of a secondregion of the heater increases from the second width to the first widthalong the direction substantially parallel to the guiding direction ofthe rib waveguide.4. The optoelectronic device of clause 1, wherein the heater comprisesplural metal strips, connected at one end to an adjacent metal strip soas to form a serpentine form.5. The optoelectronic device of clause 4, wherein the heater comprisesat least 2 metal strips and no more than 20 metal strips.6. The optoelectronic device of either clause 4 or 5, further includinga first and second electrode for the heater, which are electricallyconnected to the heater on the same side.7. The optoelectronic device of any of clauses 4-6, wherein each metalstrip has a width of at least 0.5 μm and no more than 10 μm.8. The optoelectronic device of any of clauses 4-7, wherein a gapbetween adjacent metal strips has a width of at least 0.5 μm and no morethan 10 μm.9. The optoelectronic device of any of clauses 4-8, wherein the heateris formed from any one of: Ti, TiN, TiW, NiCr, or W.10. The optoelectronic device of any preceding clause, wherein theheater is disposed above an electrical contact for the slab portion andseparated therefrom by an insulator.11. The optoelectronic device of any preceding clause, further includingan upper cladding layer disposed on the heater12. The optoelectronic device of any preceding clause, further includinga second heater, substantially identical to the first and disposed on anopposing side of the ridge portion.13. The optoelectronic device of any preceding clause, furtherincluding:

a bottom cladding layer, disposed adjacent to the slab portion; and

a thermal isolation trench, wherein the thermal isolation trench ispositioned adjacent to the bottom cladding layer.

14. The optoelectronic device of clause 13, wherein the thermalisolation trench is filled with either air or silicon dioxide.15. The optoelectronic device of either clause 13 or 14, wherein thethermal isolation trench has a width of at least 0.5 μm and no more than2.0 μm.16. The optoelectronic device of any of clauses 13-15, wherein thedevice includes plural thermal isolation trenches, which are arrangedaround a periphery of the slab portion.17. The optoelectronic device of any preceding clause, wherein thedevice further includes:

a bottom cladding layer, disposed adjacent to the slab portion; and

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

18. The optoelectronic device of clause 17, further including:

a buried oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.

19. The optoelectronic device of either clause 17 or 18, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.20. The optoelectronic device of any preceding clause, furthercomprising an electrode, electrically connected to either the ridgeportion or the slab portion, wherein the electrode includes at least onethermal isolation cavity.21. The optoelectronic device of clause 20, wherein the electrodecomprises plural thermal isolation cavities in an array, wherein thearray extends in a direction substantially parallel to the guidingdirection of the rib waveguide.22. The optoelectronic device of clause 21, wherein the array extendsfor a length of at least 50 μm and no more than 100 μm.23. The optoelectronic device of any of clauses 20-22, wherein theelectrode comprises at least 2 cavities and no more than 30 cavities.24. The optoelectronic device of any of clauses 20-23, wherein the oreach cavity in the electrode has a length of at least 2 μm and no morethan 30 μm.25. The optoelectronic device of any of clauses 20-24, wherein the oreach cavity in the electrode has a width of at least 1 μm and no morethan 10 μm.26. The optoelectronic device of any of clauses 21-23, wherein a gapbetween adjacent cavities in the electrode has a length of at least 1 μmand no more than 20 μm.27. The optoelectronic device of any of clauses 20-26, wherein the oreach cavity in the electrode is filled with air or SiO₂.28. An optoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

the device further comprising a heater, disposed in an epitaxialcrystalline cladding layer which is located beneath the slab portion.

29. The optoelectronic device of clause 28, wherein the heater comprisesa doped region of the epitaxial crystalline cladding layer.30. The optoelectronic device of clause 29, wherein the doped region ofthe epitaxial crystalline cladding layer extends in a directionsubstantially parallel to the guiding direction of the rib waveguide.31. The optoelectronic device either of clauses 29 or 30, wherein thedoped region has a width of at least 1 μm and no more than 30 μm.32. The optoelectronic device of any of clauses 29-31, wherein the dopedregion has a height of at least 0.1 μm and no more than 0.3 μm.33. The optoelectronic device of any of clauses 29-32, wherein the dopedregion has a doping concentration of at least 1×10²⁰ cm⁻³ and no morethan 2.5×10²⁰ cm⁻³.34. The optoelectronic device of any of clauses 29-33, further includingan undoped region of the epitaxial crystalline cladding layer, theundoped region being located between the doped region and the slabportion.35. The optoelectronic device of any of clauses 28-34, wherein thedevice further includes:

a thermal isolation trench, wherein the thermal isolation trench islocated adjacent to the epitaxial crystalline cladding layer.

36. The optoelectronic device of clause 35, wherein the thermalisolation trench is filled with either air or silicon dioxide.37. The optoelectronic device of either clause 35 or 36, wherein thethermal isolation trench has a width of at least 0.5 μm and no more than2.0 μm.38. The optoelectronic device of any of clauses 35-37, wherein thedevice includes plural thermal isolation trenches, which are arrangedaround a periphery of the slab portion.39. The optoelectronic device of any of clauses 28-38, wherein thedevice further includes:

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

40. The optoelectronic device of clause 39, further including:

a buried oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.

41. The optoelectronic device of either clause 38 or 40, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.42. The optoelectronic device of any of clauses 28-41, further includingan electrode, electrically connected to either the ridge or the slabportion, wherein the electrode includes at least one thermal isolationcavity.43. The optoelectronic device of clause 42, wherein the electrodecomprises plural thermal isolation cavities in an array, wherein thearray extends in a direction substantially parallel to the guidingdirection of the rib waveguide.44. The optoelectronic device of clause 43, wherein the array extendsfor a length of at least 50 μm and no more than 100 μm.45. The optoelectronic device of any of clauses 42-44, wherein theelectrode comprises at least 2 cavities and no more than 30 cavities.46. The optoelectronic device of any of clauses 42-45, wherein the oreach cavity in the electrode has a length of at least 2 μm and no morethan 30 μm.47. The optoelectronic device of any of clauses 42-46, wherein the oreach cavity in the electrode has a width of at least 1 μm and no morethan 10 μm.48. The optoelectronic device of any of clauses 43-45, wherein a gapbetween adjacent cavities in the electrode has a length of at least 1 μmand no more than 20 μm.49. The optoelectronic device of any of clauses 42-48, wherein the oreach cavity in the electrode if filled with air or SiO₂.50. An optoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

the device further comprising:

a heater, for heating the temperature-sensitive optically active region;

a bottom cladding layer, disposed adjacent to the slab portion;

and a thermal isolation trench, wherein the thermal isolation trench ispositioned adjacent to the bottom cladding layer.

51. The optoelectronic device of clause 50, wherein the thermalisolation trench is filled with either air or silicon dioxide.52. The optoelectronic device of either of clauses 50 or 51, wherein thethermal isolation trench has a width of at least 0.5 μm and no more than2.0 μm.53. The optoelectronic device of any of clauses 50-52, including pluralthermal isolation trenches, which are arranged around a periphery of theslab portion.54. The optoelectronic device of any of clauses 50-53, wherein theheater is disposed on top of the slab portion, and wherein a part of theheater closest to the ridge portion is at least 2 μm away from the ridgeportion.55. The optoelectronic device of clause 54, wherein a width of a firstregion of the heater tapers from a first width to a second width in adirection substantially parallel to a guiding direction of the ribwaveguide.56. The optoelectronic device of clause 55, wherein the width of asecond region of the heater increases from the second width to the firstwidth along the direction substantially parallel to the guidingdirection of the rib waveguide.57. The optoelectronic device of clause 54, wherein the heater comprisesplural metal strips, connected at one end to an adjacent metal strip soas to form a serpentine form.58. The optoelectronic device of clause 57, wherein the heater comprisesat least 2 metal strips and no more than 20 metal strips.59. The optoelectronic device of either clause 57 or 58, furtherincluding a first and second electrode for the heater, which areelectrically connected to the heater on the same side.60. The optoelectronic device of any of clauses 57-59, wherein eachmetal strip has a width of at least 0.5 μm and no more than 10 μm.61. The optoelectronic device of any of clauses 57-60, wherein a gapbetween adjacent metal strips has a width of at least 0.5 μm and no morethan 10 μm.62. The optoelectronic device of any of clauses 57-61, wherein theheater is formed from any one of Ti, TiN, TiW, NiCr, or W.63. The optoelectronic device of any of clauses 54-62, wherein theheater is disposed above an electrical contact for the slab portion andseparated therefrom by an insulator.64. The optoelectronic device of any of clauses 54-63, further includingan upper cladding layer disposed on the heater.65. The optoelectronic device of any of clauses 54-64, further includinga second heater, substantially identical to the first and disposed on anopposing side of the ridge portion.66. The optoelectronic device of any of clauses 50-52, wherein theheater is disposed in the bottom cladding layer which is an epitaxialcrystalline cladding layer.67. The optoelectronic device of clause 66, wherein the heater comprisesa doped region of the epitaxial crystalline cladding layer.68. The optoelectronic device of clause 67, wherein the doped region ofthe epitaxial crystalline cladding layer extends in a directionsubstantially parallel to the guiding direction of the rib waveguide.69. The optoelectronic device of either clause 67 or 68, wherein thedoped region has a width of at least 1 μm and no more than 30 μm.70. The optoelectronic device of any of clauses 67-69, wherein the dopedregion has a height of at least 0.1 μm and no more than 0.3 μm.71. The optoelectronic device of any of clauses 67-70, wherein the dopedregion has a doping concentration of at least 1×10²⁰ cm⁻³ and no morethan 2.5×10²⁰ cm⁻³.72. The optoelectronic device of any of clauses 67-71, further includingan undoped region of the epitaxial crystalline cladding layer, theundoped region being located between the doped region and the slabportion.73. The optoelectronic device of clauses 50-72, wherein the devicefurther includes:

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

74. The optoelectronic device of clause 73, further including:

a buried oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.

75. The optoelectronic device of either clause 73 or 74, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.76. The optoelectronic device of clauses 50-75, further comprising anelectrode, electrically connected to either the ridge portion or theslab portion, wherein the electrode includes at least one thermalisolation cavity.77. The optoelectronic device of clause 76, wherein the electrodecomprises plural thermal isolation cavities in an array, wherein thearray extends in a direction substantially parallel to the guidingdirection of the rib waveguide.78. The optoelectronic device of clause 77, wherein the array extendsfor a length of at least 50 μm and no more than 100 μm.79. The optoelectronic device of any of clauses 76-78, wherein theelectrode comprises at least 2 cavities and no more than 30 cavities.80. The optoelectronic device of any of clauses 76-79, wherein the oreach cavity in the electrode has a length of at least 2 μm and no morethan 30 μm.81. The optoelectronic device of any of clauses 76-80, wherein the oreach cavity in the electrode has a width of at least 1 μm and no morethan 10 μm.82. The optoelectronic device of any of clauses 77-79, wherein a gapbetween adjacent cavities in the electrode has a length of at least 1 μmand no more than 20 μm.83. The optoelectronic device of any of clauses 76-82, wherein the oreach cavity in the electrode is filled with air or SiO₂.84. An optoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

wherein the device further comprises:

a heater, for heating the temperature-sensitive optically active region;

a bottom cladding layer, disposed adjacent to the slab portion; and

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

85. The optoelectronic device of clause 84, further including:

a buried oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.

86. The optoelectronic device of either clause 84 or 85, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.87. The optoelectronic device of any of clauses 84-86, further includinga thermal isolation trench, wherein the thermal isolation trench ispositioned adjacent to the bottom cladding layer.88. The optoelectronic device of clause 87, wherein the thermalisolation trench is filled with either air or silicon dioxide.89. The optoelectronic device of either of clauses 87 or 88, wherein thethermal isolation trench has a width of at least 0.5 μm and no more than2.0 μm.90. The optoelectronic device of any of clauses 87-89, including pluralthermal isolation trenches, which are arranged around a periphery of theslab portion.91. The optoelectronic device of any of clauses 87-90, wherein theheater is disposed on top of the slab portion, and wherein a part of theheater closest to the ridge portion is at least 2 μm away from the ridgeportion.92. The optoelectronic device of clause 91, wherein a width of a firstregion of the heater tapers from a first width to a second width in adirection substantially parallel to a guiding direction of the ribwaveguide.93. The optoelectronic device of clause 92, wherein the width of asecond region of the heater increases from the second width to the firstwidth along the direction substantially parallel to the guidingdirection of the rib waveguide.94. The optoelectronic device of clause 91, wherein the heater comprisesplural metal strips, connected at one end to an adjacent metal strip soas to form a serpentine form.95. The optoelectronic device of clause 94, wherein the heater comprisesat least 2 metal strips and no more than 20 metal strips.96. The optoelectronic device of either clause 94 or 95, furtherincluding a first and second electrode for the heater, which areelectrically connected to the heater on the same side.97. The optoelectronic device of any of clauses 94-96, wherein eachmetal strip has a width of at least 0.5 μm and no more than 10 μm.98. The optoelectronic device of any of clauses 94-97, wherein a gapbetween adjacent metal strips has a width of at least 0.5 μm and no morethan 10 μm.99. The optoelectronic device of any of clauses 94-98, wherein theheater is formed from any one of Ti, TiN, TiW, NiCr, or W.100. The optoelectronic device of any of clauses 90-99, wherein theheater is disposed above an electrical contact for the slab portion andseparated therefrom by an insulator.101. The optoelectronic device of any of clauses 90-100, furtherincluding an upper cladding layer disposed on the heater.102. The optoelectronic device of any of clauses 90-100, furtherincluding a second heater, substantially identical to the first anddisposed on an opposing side of the ridge portion.103. The optoelectronic device of any of clauses 84-89, wherein theheater is disposed in the bottom cladding layer which is an epitaxialcrystalline cladding layer.104. The optoelectronic device of clause 100, wherein the heatercomprises a doped region of the epitaxial crystalline cladding layer.105. The optoelectronic device of clause 104, wherein the doped regionof the epitaxial crystalline cladding layer extends in a directionsubstantially parallel to the guiding direction of the rib waveguide.106. The optoelectronic device of either clause 104 or 105, wherein thedoped region has a width of at least 1 μm and no more than 30 μm.107. The optoelectronic device of any of clauses 104-106, wherein thedoped region has a height of at least 0.1 μm and no more than 0.3 μm.108. The optoelectronic device of any of clauses 104-107, wherein thedoped region has a doping concentration of at least 1×10²⁰ cm⁻³ and nomore than 2.5×10^(°)cm⁻³.109. The optoelectronic device of any of clauses 104-108, furtherincluding an undoped region of the epitaxial crystalline cladding layer,the undoped region being located between the doped region and the slabportion.110. The optoelectronic device of clauses 83-109, further comprising anelectrode, electrically connected to either the ridge portion or theslab portion, wherein the electrode includes at least one thermalisolation cavity.111. The optoelectronic device of clause 110, wherein the electrodecomprises plural thermal isolation cavities in an array, wherein thearray extends in a direction substantially parallel to the guidingdirection of the rib waveguide.112. The optoelectronic device of clause 111, wherein the array extendsfor a length of at least 50 μm and no more than 100 μm.113. The optoelectronic device of any of clauses 110-112, wherein theelectrode comprises at least 2 cavities and no more than 30 cavities.114. The optoelectronic device of any of clauses 110-113, wherein the oreach cavity in the electrode has a length of at least 2 μm and no morethan 30 μm.115. The optoelectronic device of any of clauses 110-114, wherein the oreach cavity in the electrode has a width of at least 1 μm and no morethan 10 μm.116. The optoelectronic device of any of clauses 111-113, wherein a gapbetween adjacent cavities in the electrode has a length of at least 1 μmand no more than 20 μm.117. The optoelectronic device of any of clauses 110-116, wherein the oreach cavity in the electrode is filled with air or SiO₂.118. An optoelectronic device, including:

a rib waveguide, the rib waveguide including:

-   -   a ridge portion, which includes a temperature-sensitive        optically active region,    -   and a slab portion, positioned adjacent to the ridge portion;

wherein the device further comprises:

a heater, for heating the temperature-sensitive optically active region;and

an electrode, electrically connected to either the ridge portion or theslab portion, and a heater positioned between the electrode and theridge portion;

wherein the electrode includes at least one thermal isolation cavity.

119. The optoelectronic device of clause 118, wherein the electrodecomprises plural thermal isolation cavities in an array, wherein thearray extends in a direction substantially parallel to the guidingdirection of the rib waveguide.120. The optoelectronic device of clause 119, wherein the array extendsfor a length of at least 50 μm and no more than 100 μm.121 The optoelectronic device of any of clauses 118-120, wherein theelectrode comprises at least 2 cavities and no more than 30 cavities.122. The optoelectronic device of any of clauses 118-121, wherein the oreach cavity in the electrode has a length of at least 2 μm and no morethan 30 μm.123. The optoelectronic device of any of clauses 118-122, wherein the oreach cavity in the electrode has a width of at least 1 μm and no morethan 10 μm.124. The optoelectronic device of any of clauses 119-121, wherein a gapbetween adjacent cavities in the electrode has a length of at least 1 μmand no more than 20 μm.125. The optoelectronic device of any of clauses 118-124, wherein the oreach cavity in the electrode is filled with air or SiO₂.126. The optoelectronic device of any of clauses 118-125, furtherincluding

a bottom cladding layer, disposed adjacent to the slab portion; and

a thermal isolation cavity, located on an opposing side of the bottomcladding layer to the slab portion.

127. The optoelectronic device of clause 126, further including:

a buried oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.

128. The optoelectronic device of either clause 126 or 127, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.129. The optoelectronic device of any of clauses 126-128, furtherincluding a thermal isolation trench, wherein the thermal isolationtrench is positioned adjacent to the bottom cladding layer.130. The optoelectronic device of clause 129, wherein the thermalisolation trench is filled with either air or silicon dioxide.131. The optoelectronic device of either of clauses 129 or 130, whereinthe thermal isolation trench has a width of at least 0.5 μm and no morethan 2.0 μm.132. The optoelectronic device of any of clauses 129-131, includingplural thermal isolation trenches, which are arranged around a peripheryof the slab portion.133. The optoelectronic device of any of clauses 118-132, wherein theheater is disposed on top of the slab portion, and wherein a part of theheater closest to the ridge portion is at least 2 μm away from the ridgeportion.134. The optoelectronic device of clause 133, wherein a width of a firstregion of the heater tapers from a first width to a second width in adirection substantially parallel to a guiding direction of the ribwaveguide.135. The optoelectronic device of clause 134, wherein the width of asecond region of the heater increases from the second width to the firstwidth along the direction substantially parallel to the guidingdirection of the rib waveguide.136. The optoelectronic device of clause 133, wherein the heatercomprises plural metal strips, connected at one end to an adjacent metalstrip so as to form a serpentine form.137. The optoelectronic device of clause 136, wherein the heatercomprises at least 2 metal strips and no more than 20 metal strips.138. The optoelectronic device of either clause 136 or 137, furtherincluding a first and second electrode for the heater, which areelectrically connected to the heater on the same side.139. The optoelectronic device of any of clauses 136-138, wherein eachmetal strip has a width of at least 0.5 μm and no more than 10 μm.140. The optoelectronic device of any of clauses 136-139, wherein a gapbetween adjacent metal strips has a width of at least 0.5 μm and no morethan 10 μm.141. The optoelectronic device of any of clauses 136-140, wherein theheater is formed from any one of Ti, TiN, TiW, NiCr, or W.142. The optoelectronic device of any of clauses 133-141, wherein theheater is disposed above an electrical contact for the slab portion andseparated therefrom by an insulator.143. The optoelectronic device of any of clauses 133-143, furtherincluding an upper cladding layer disposed on the heater.144. The optoelectronic device of any of clauses 133-144, furtherincluding a second heater, substantially identical to the first anddisposed on an opposing side of the ridge portion.145. The optoelectronic device of any of clauses 118-132, wherein theheater is disposed in the bottom cladding layer which is an epitaxialcrystalline cladding layer.146. The optoelectronic device of clause 145, wherein the heatercomprises a doped region of the epitaxial crystalline cladding layer.147. The optoelectronic device of clause 146, wherein the doped regionof the epitaxial crystalline cladding layer extends in a directionsubstantially parallel to the guiding direction of the rib waveguide.148. The optoelectronic device of either clause 146 or 147, wherein thedoped region has a width of at least 1 μm and no more than 30 μm.149. The optoelectronic device of any of clauses 146-148, wherein thedoped region has a height of at least 0.1 μm and no more than 0.3 μm.150. The optoelectronic device of any of clauses 146-149, wherein thedoped region has a doping concentration of at least 1×10²⁰ cm⁻³ and nomore than 2.5×10²⁰ cm⁻³.151. The optoelectronic device of any of clauses 146-150, furtherincluding an undoped region of the epitaxial crystalline cladding layer,the undoped region being located between the doped region and the slabportion.

1.-20. (canceled)
 21. An optoelectronic device, including: a ribwaveguide, the rib waveguide including: a ridge portion, which includesa temperature-sensitive optically active region, and a slab portion,positioned adjacent to the ridge portion; wherein the optoelectronicdevice further comprises: a heater, for heating thetemperature-sensitive optically active region; a bottom cladding layer,disposed adjacent to the slab portion; and a thermal isolation cavity,located on an opposing side of the bottom cladding layer to the slabportion.
 22. The optoelectronic device of claim 21, further including: aburied oxide layer, disposed adjacent to a lower surface of the bottomcladding layer, wherein the thermal isolation cavity is located on anopposing side of the buried oxide layer and is adjacent to a siliconsubstrate.
 23. The optoelectronic device of claim 21, wherein thethermal isolation cavity has a width which is larger than a width of theslab portion.
 24. The optoelectronic device of claim 21, furtherincluding a thermal isolation trench, wherein the thermal isolationtrench is positioned adjacent to the bottom cladding layer.
 25. Theoptoelectronic device of claim 24, wherein the heater is disposed on topof the slab portion, and wherein a part of the heater closest to theridge portion is at least 2 μm away from the ridge portion.
 26. Theoptoelectronic device of claim 25, wherein a width of a first region ofthe heater tapers from a first width to a second width in a directionsubstantially parallel to a guiding direction of the rib waveguide. 27.The optoelectronic device of claim 26, wherein the width of a secondregion of the heater increases from the second width to the first widthalong the direction substantially parallel to the guiding direction ofthe rib waveguide.
 28. The optoelectronic device of claim 25, whereinthe heater comprises plural metal strips, connected at one end to anadjacent metal strip so as to form a serpentine form.
 29. Theoptoelectronic device of claim 25, further including a first and secondelectrode for the heater, which are electrically connected to the heateron the same side.
 30. The optoelectronic device of claim 28, wherein theheater is formed from any one of Ti, TiN, TiW, NiCr, or W.
 31. Theoptoelectronic device of claim 24, wherein the heater is disposed abovean electrical contact for the slab portion and separated therefrom by aninsulator.
 32. The optoelectronic device of claim 24, further includingan upper cladding layer disposed on the heater.
 33. The optoelectronicdevice of claim 24, wherein the heater is a first heater, theoptoelectronic device further including a second heater, substantiallyidentical to the first heater and disposed on an opposing side of theridge portion.
 34. The optoelectronic device of claim 21, wherein theheater is disposed in the bottom cladding layer which is an epitaxialcrystalline cladding layer.
 35. The optoelectronic device of claim 34,wherein the heater comprises a doped region of the epitaxial crystallinecladding layer.
 36. The optoelectronic device of claim 35, wherein thedoped region of the epitaxial crystalline cladding layer extends in adirection substantially parallel to the guiding direction of the ribwaveguide.
 37. The optoelectronic device of claim 35, further includingan undoped region of the epitaxial crystalline cladding layer, theundoped region being located between the doped region and the slabportion.
 38. The optoelectronic device of claim 21, further comprisingan electrode, electrically connected to either the ridge portion or theslab portion, wherein the electrode includes at least one thermalisolation cavity.
 39. The optoelectronic device of claim 38, wherein theelectrode comprises plural thermal isolation cavities in an array,wherein the array extends in a direction substantially parallel to theguiding direction of the rib waveguide.
 40. The optoelectronic device ofclaim 39, wherein the or each cavity in the electrode is filled with airor SiO2.